Field of the Invention
A work vehicle equipped with a hydro-static transmission (HST) is disclosed.
Background Information
A crawler-type hydraulic drive vehicle equipped with a conventional hydro-static transmission is configured to prevent overrunning thereof in down slope movement by predicting the increased speed thereof and controlling the displacement of a hydraulic pump to cancel out the predicted increased speed (see e.g., Japan Laid-open Patent Application Publication No. H06-191317).
However, the cost required for the crawler-type hydraulic drive vehicle becomes quite high when the hydraulic pump is designed to have a large displacement to cope with all speeds and gradients of slopes that are assumable in movement of the crawler-type hydraulic drive wheel. Thus in reality, it is preferable to set the hydraulic pump to have a displacement enough to cope with speeds and gradients of slopes at which the vehicle is operated with high frequency, and on the premise of the setting, preferable to employ an overrun control aside from an increase in displacement of the hydraulic pump.
A work vehicle in accordance with exemplary embodiments of the present invention is equipped with a hydro-static transmission whereby even when an increase in displacement of a hydraulic pump is limited, overrunning is controllable by another means.
A work vehicle according to a first aspect includes an engine; a fan variable displacement pump and a first travelling variable displacement pump; a first travelling variable displacement motor, a drive unit; a fan motor; a fan; an overrun state detecting unit; and a control unit. The fan variable displacement pump and the first travelling variable displacement pump are configured to be driven by the engine. The first travelling variable displacement motor is configured to be rotated by a pressurized oil of the first travelling variable displacement pump. The drive unit is configured to be driven by the first travelling variable displacement motor. The fan motor is configured to be rotated by a pressurized oil of the fan variable displacement pump. The fan is configured to be driven by the fan motor.
The overrun state detecting unit is configured to detect an overrun state of the drive unit. The control unit is configured to control a rotational speed of the fan to reach a first fan rotational speed by increasing the rotational speed of the fan when a detection value detected by the overrun state detecting unit becomes greater than or equal to a first threshold. The control unit is configured to control the rotational speed of the fan to reach a second fan rotational speed higher than the first fan rotational speed when the detection value becomes greater than or equal to a second threshold greater than the first threshold.
The overrun state detecting unit may include an engine rotational speed detecting unit configured to detect a rotational speed of the engine. The detection value may be the engine rotational speed. The first threshold may be a first engine rotational speed. The second threshold may be a second engine rotational speed higher than the first engine rotational speed.
The first engine rotational speed may be higher than a rated rotational speed. The first engine rotational speed may be higher than a high idle rotational speed.
The present work vehicle may further include a vehicle speed setting unit and a vehicle speed measuring unit. The vehicle speed setting unit may be configured to set a target vehicle speed based on an operating signal detected from an operating member. The vehicle speed measuring unit may be configured to measure an actual speed of the work vehicle. The control unit may be configured to terminate controlling the rotational speed of the fan to reach either the first fan rotational speed or the second fan rotational speed when the actual vehicle speed becomes less than or equal to a predetermined vehicle speed less than or equal to the target vehicle speed.
The second fan rotational speed may be higher than an upper rotational speed limit of the fan to be applied when the engine rotational speed is lower than the first engine rotational speed.
A difference between the second engine rotational speed and the first engine rotational speed may be greater than or equal to 50 rpm and less than or equal to 300 rpm.
A difference between the second fan rotational speed and the first fan rotational speed may be greater than or equal to 250 rpm and less than or equal to 600 rpm.
The aforementioned work vehicle may further include a second travelling variable displacement pump and a second travelling variable displacement motor. The second travelling variable displacement pump may be configured to be driven by the engine. The second travelling variable displacement motor may be configured to be rotated by a pressurized oil of the second travelling variable displacement pump. The drive unit may be configured to be driven by the second travelling variable displacement motor. The drive unit may include right and left crawler belts and right and left sprockets respectively configured to drive the right and left crawler belts. The first travelling variable displacement motor may be configured to drive one of the right and left sprockets. The second travelling variable displacement motor may be configured to drive the other of the right and left sprockets.
The first fan rotational speed and the second fan rotational speed may be predetermined constant values.
The aforementioned work vehicle may include a cab disposed rearward of the engine. The fan may be disposed rearward of and adjacently to the cab.
In the work vehicle according to the first aspect, in loads applied to the variable displacement pumps due to overrunning, the amount of load to be absorbed by the fan variable displacement pump increases. As a result, overrunning is prevented. Additionally, the fan rotational speed is controlled to increase in a stepwise manner in accordance with the state of overrunning. Therefore, when the extent of overrunning is relatively small, the sound of the fan does not increase in volume. When the extent of overrunning is relatively large, overrunning is reliably prevented, and simultaneously, the sound of the fan does not suddenly increase in volume. Therefore, the comfortableness of an operator can be maintained.